Automotive fire apparatus (often referred to as “fire trucks” or “fire engines”) typically include a centrifugal pump that provides the water pressure to fire hoses for fire fighters while they attempt to eliminate fires.
Because these pumps are so critical to the fire-fighting effort, this equipment must be maintained properly.
The National Fire Protection Association (NFPA) has promulgated standards for the inspection, maintenance, testing, and retirement of in-service automotive fire apparatus since at least as far back as 1987.
Beginning in 1991, the NFPA began issuing standards which were more proactive than in previous years. For example, the 1991 standards specified that 3″ or larger valves be tested at 500 psi, that intake relief valves be used to manage incoming pressure, that 30-degree sweep elbows be provided on the discharges to eliminate hose kinking, and that all three-inch and larger discharges be eliminated from the pump panel to reduce the possibility of injuries to pump operators.
The NFPA's most recently released standard is entitled “NFPA 1911: Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, 2012 Edition” (“NFPA 1911”) which is incorporated herein in full.
This latest standard includes a long list of checks and tests by which operators of fire engines can ensure that their apparatus are kept in proper working order. These tests are to be conducted at least annually, or when any substantial repair or modification is made.
One of the important tests conducted concerns the strength and proper operation of the pumps on the apparatus. This test, known colloquially as the “Draft Test”, tests the ability of a pump to pull water from a source below the apparatus and provide a source of pressurized water for fire fighters.
The Draft Test can be difficult to perform, as the apparatus should be parked near a water source, and positioned such that the vertical distance between the water source and pump intake is defined in the test (usually ten feet), based on the pump capacity. (See NFPA 1911, Chapter 18.)
Alternatively, the test can be conducted from a hydrant, but the test can easily use more than 50,000 gallons, which can impact municipal water sources. The tester must take the hydrant pressure into account. Finally, use of municipal water means that operators must plan for the discharge of the considerable amount of water, which can be alarming to those nearby. For these reasons, use of municipal water hydrants is disfavored by those who conduct these tests.
To move the apparatus to a testing area inherently means that a unit is out of service longer than it would be in comparison to a test that could be conducted at a fire station.
Test operations require flow measurements using pitot tubes accurate to 5%, constant monitoring to prevent nozzle movement. Instruments measuring pressure must meet ASME B40.100 (Pressure Gauges and Gauge Attachments), which the inventor herein incorporates. This standard requires test gauges to be at least 3.5″ and a range of 30 in. Hg (100 kPa) vacuum to zero for a vacuum gauge (or 30 in. to 150 psi for a compound gauge).
Because a typical test in the current art requires use of a pitot gauge to get an accurate reading, the discharge water stream is positioned so the water flows into the discharge water tank from above the surface of the water. The churn of the discharge water aerates the water. If the system in use is a closed-cycle, the aerated water can hide a vacuum leak in the pump. Eliminating air in the resource/recovery tank would be beneficial for that purpose.
These tests include many other requirements, such as measurement of engine speed within 50 rpm during the tests, the ambient air temperature, water temperature, elevation of the test site, and atmospheric pressure, must all be recorded before and after the pump testing.
All these instruments must be calibrated within 60 days preceding the tests, and as the standard changes, fire station personnel must make changes to reflect those standards.
If a pump will be tested from draft (directly from a water source), the pump must be “primed” by use of a positive displacement pump or a air priming system which lowers atmospheric pressure within the pump allowing water to pushed into the from a higher outside atmospheric pressure.
Because of the foregoing reasons and other requirements included in NFPA 1911 and ASME B40.100, fire departments continue to seek better ways to test the centrifugal pumps on their apparatus without sacrificing the quality of a true draft test. This statement holds true for any industry searching for a more efficient and accurate flow test. Those who are seeking these flow tests need a system that performs with such efficiency that there is no excessive down time, keeping their instruments calibrated, conducting their test without impacting the municipal water supplies, and without concerning themselves with discharged water runoff.